Altimeters do not measure accurately the height above the ground. It is only a relative number. The altimeter has a very sensitive pressure reading device called an aneroid. It is a series of hollow disk

Both. Look at it logically. Why would they lay a marble runways when concrete runways are more practical for many ways. Lets start with with being seamless. In all the years I have flown I never landed

Any smooth hard surface can be used for a runway. The cost of marble runway would be very expensive and not practical. Marble is not a strong material so it would have to be thick. that would add to the